At the begining, it failed because my node cache wasn't high enough. After I increased it, I've read it was getting killed because I needed 40G ram without --slim, so I used slim. Then I've read it was killed because I needed --flat-nodes with --slim, so I used --flat-nodes.
I can't find what I am doing wrong.
I'm sure it is obvious for some of you what I am doing wrong.
If you can help me, I would realy appreciate it.
Thanks!

... and (just in case you haven't already done this) I'd definitely run the process all the way through on a smaller extract first - and obviously if you don't have a need to render the whole planet, it makes sense to use a smaller extract anyway.

As scai has pointed out you are likely running out of memory, you will need to add swap (which while slower is still preferrable to any other solution). Further I would suggest not running with 8 threads which, given the size of your machine, is likely to be counter productive.